Rapid thermal processing of semiconductor wafers is well known, and the need for a wafer support fixture having a low effective thermal mass, chemical stability in the presence of extremely corrosive conditions, and physical stability when exposed to high temperatures and rapid thermal cycling over an extended period of time, has been recognized. More specifically, it has also been recognized that silicon carbide is a superior material for construction of such fixtures. See for example U.S. Pat. No. 4,978,567 issued to Michael Miller on Dec. 18, 1990, incorporated herein by reference.
The fixture of the Miller patent consists of silicon carbide, and is fabricated by chemical vapor deposition of the carbide on a graphite substrate, followed by destructive oxidation to remove the graphite. Note particularly that the Miller fixture is a single piece of silicon carbide, including a wafer support surface formed integrally with an annular surface surrounding the wafer support, and further including an annular sidewall, integral therewith and perpendicular to the wafer support surface, for holding the wafer support surface at the proper height.
In the Miller fabrication method, the graphite interface with deposited silicon carbide is always formed on the back side of the wafer support section, opposite the support face designed for contact with the semiconductor wafer. As a result, there is no convenient technique for providing such a wafer support face with a precisely planar finish. Also, the Miller process does not allow the mold to be used for providing precisely detailed structural features in the support face.
Other known fixtures are composed of high-purity graphite, coated with a thin layer of CVD silicon carbide. The coating is required in order to seal the surface of the graphite, so that the graphite cannot contaminate the reactor system. Grinding of the coated surface may or may not be required, depending upon the intricacy of surface features, and the degree of precision desired.
Such coated graphite fixtures have known disadvantages, including a limited lifetime due to stresses developed during thermal cycling, because the silicon carbide coating has a different coefficient of thermal expansion (CTE), compared with the graphite. Long before the coating develops visible cracks, it begins to "leak" and thereby allow the underlying graphite to contaminate the system. If thicker coatings are applied in an attempt to avoid the problem, the cracks appear even sooner.
Despite these disadvantages, the coated graphite fixtures have found some degree of success, particularly when used in reactors equipped with induction heating, since graphite has the electrical conductivity required to enable use of these support sections as susceptors for induction heating. Thus, any material selected to replace coated graphite in such fixtures should preferably exhibit all the desirable features of silicon carbide, and also have the conductivity needed for inductive heating.
Recent advances in rapid thermal processing have also led to the requirement that the wafer support section of a fixture be as thin as possible, so that more rapid thermal cycling can be achieved. But the flexible character of graphite places a severe limitation upon the use of coated graphite for thinner support sections, since thermal processing and repeated thermal cycling will cause such support sections to lose planarity. Pure silicon carbide will not lose its planarity, even when thinned to 20 mils or less.